In the optical communications space, receivers based on coherent detection techniques have suffered disadvantages that have, to date, prevented successful deployment in “real-world” installed communications networks.
For example, coherent optical receivers tend to be highly sensitive to optical impairments of the received carrier signal. Optical signals received through conventional optical links are typically distorted by significant amounts of chromatic dispersion (CD) and polarization dependent impairments such as Polarization Mode Dispersion (PMD), polarization angle changes and polarization dependent loss (PDL). Chromatic dispersion (CD) on the order of 30,000 ps/nm, and polarization rotation transients at rates of 105 Hz are commonly encountered.
Various methods and systems intended to address some of these limitations are known in the art. For example, a method of compensating polarization angle impairments are described in PLL-Free Synchronous QPSK Polarization Multipex/Diversity Receiver Concept with Digital I&Q Baseband Processing, R Noé, IEEE Photonics Technology Letters, Vol. 17, No. 4, April 2005. In the introduction of this same paper, Noé also alludes to the possibility of also compensating chromatic dispersion, but provides no further discussion as to how this might be done.
Applicant's co-pending U.S. patent application Ser. No. 09/975,985, entitled Measurement Of Polarization Dependent Loss In An Optical Transmission System; and Applicant's U.S. Pat. No. 6,760,149, entitled Compensation Of Polarization Dependent Loss, the content of which are hereby incorporated herein by reference, teach methods of measuring and compensating PDL in an optical communications system. The techniques described in these references are particularly suited to “real world” network installations, as opposed to laboratory simulations.
Known methods of compensating Polarization Dependent Loss (PDL) typically involve detecting the polarization state of the received optical signal, and then controlling one or more polarization rotators and/or variable optical attenuators (VOAs) to adjust the power level of each of the orthogonal polarizations. In laboratory systems, the polarization rotators and/or VOAs are often manually adjusted, which clearly does not provide a useful arrangement for real-world network installation. Automatic systems, for example as described in Applicant's U.S. Pat. No. 6,760,149, address this issue by implementing an adaptive control loop for driving the VOAs. However automated PDL compensation systems typically suffer a limitation in that the received optical signal must have certain known optical characteristics. At a minimum, the received optical signal must necessarily be a valid optical signal, rather than Amplified Spontaneous Emission (ASE) and thermal noise, for example. In many systems, it is also necessary that the received optical signal be composed of a pair of orthogonal polarizations.
However, in “real-world” network installations, neither of these criteria can be guaranteed. For example, following initial installation of an optical link, the only light in the optical fiber may be ASE and thermal noise. This state may persist for an extended period of time. Even in the absence of light in the fiber, there may be a significant DC offset or dark current in one or more parts of a receiver, which might capture an adaptive control loop. In other cases, optical signals that may be present at the input yet occupy optical frequencies significantly different to that of LO can add further noise and offset. For example such a signal may be centered at multiples of 50 GHz away from the nominal frequency of the LO. While the AC portion may be low-pass filtered out at the receiver input, the DC and noise portions can significantly affect the performance of an analog control loop.
Even when a valid signal is present in the link, the polarization impairments of the link may well be such that the transmitted polarizations are no-longer orthogonal when they reach the receiver. In addition, polarization rotation transients may well exceed the performance of a polarization rotator, thereby defeating any PDL compensation system that relies on a polarization rotator for part of its functionality. Polarization elements of the receiver that are nominally orthogonal or nominally equal will typically exhibit performance variations due to manufacturing, temperature, and aging effects.
Accordingly, cost-effective techniques for at least partially compensating effects of Polarization Dependent Loss (PDL) in a coherent optical receiver remain highly desirable.